python311Packages.av: pin to ffmpeg 5

```
av> src/av/container/core.c: In function ‘__pyx_pymod_exec_core’:
av> src/av/container/core.c:9071:36: error: ‘AVFMT_FLAG_PRIV_OPT’ undeclared (first use in this function); did you mean ‘AVFMT_FLAG_IGNIDX’?
av> 9071 | __pyx_t_3 = __Pyx_PyInt_From_int(AVFMT_FLAG_PRIV_OPT); if (unlikely(!__pyx_t_3)) __PYX_ERR(0, 160, __pyx_L1_error)
av> | ^~~~~~~~~~~~~~~~~~~
av> | AVFMT_FLAG_IGNIDX
av> src/av/container/core.c:9071:36: note: each undeclared identifier is reported only once for each function it appears in
av> error: command '/nix/store/6dz7ahcr1ybhjwzsfdjqjfypmb7jg5qf-gcc-wrapper-12.3.0/bin/gcc' failed with exit code 1
```

pkgs/development/python-modules/av/default.nix

···
 , setuptools
 
 # runtime
-, ffmpeg-headless
+, ffmpeg_5-headless
 
 # tests
 , numpy
···
   ];
 
   buildInputs = [
-    ffmpeg-headless
+    ffmpeg_5-headless
   ];
 
   preCheck = ''